Advanced Content Filter (ACF) is a CKEditor core feature that filters incoming HTML content by transforming and deleting disallowed elements, attributes, classes and styles. If you paste content into CKEditor 4 and notice that some elements are removed, then chances are high that it was removed by ACF.
Allow everything (disable ACF)
config.allowedContent = true;
// Allow <dl>, <dt>, <dd>.
config.extraAllowedContent = ‘dl dt dd’;
// Disallow setting borders for images. ‘*’ is used as a wildcard. config.disallowedContent = ‘img{border*};
// Disallow setting a target for links.
config.disallowedContent = ‘a[target]’;
// Example: Allow <u>, <em>, <strong>, <ul>, <li>, <a>. // For <a> elements, “href” attribute is required and “target” is optional.
config.allowedContent = ‘u em strong ul li;a[!href,target]’;
